Anti-collision system



Oct. 17, 1961 E. M. GOODELL ETAL 3,

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ANTICOLLISION SYSTEM 12 Sheets-Sheet 8 Filed April 17, 1958 o W O R\G\HTHAND 2 Ra HT KNEL I UWLL S'H Mu LATORS FAR RANGE LU -r HAND a LEFT KNEE.TACTILE STIMULATORB QoLusIoN WARN\NG (1 16 v 6) 51/52577- M GOODELL5440/2 LA A To 5 INVENTORS A 17-0/2 NE y:

Oct. 17, 1961 E. M.'GOODELL ETAL 3,005,194

ANTI-COLLISION SYSTEM 12 Sheets-Sheet 10 Filed April 17, 1958 1961 E. M.GOODELL ETAL 3,005,194

ANTI-COLLISION SYSTEM 12 Sheets-Sheet 11 Filed April 17, 1958 1961 E. M.GOODELL ETAL 5,005,194

ANTICOLLISION SYSTEM Filed April 17, 1958 12 Sheets-Sheet 12 AZIMUTHALFlELD PA'ITERN [MERE/7' M. GOODELL Ema/e LA/(A 7"05 INVENTORS ELEVAT\ONF\F LD PATT'ERN I 5 l a United States Patent Ofice 3,005,194 PatentedOct. 17, 1961 3,005,194 ANTI-COLLISION SYSTEM Everett M. Goodell, PalosVerdes Estates, and Emory Lakatos, Santa Monica, Calif., assignors, bymesne assignments, to Thompson Ramo Wooldridge Inc., Cleveland, Ohio, acorporation of Ohio Filed Apr. 17, 1958, Ser. No. 729,119 8 Claims. (Cl.3437) This invention relates to an improved signal analyzing method andsystem and, in particular, to an improved method and system fordetecting the presence of an object and properly indicating a collisionavoidance maneuver. While not limited thereto, the invention is hereindescribed as embodied in a novel aircraft collision avoidancearrangement for (a) sensing a collision threat (the threat may forexample be a nearby aircraft), (b) predicting the course of this threatrelative to the observer, and (c) if the threat is on a collision coursewith the observer, providing information indicative of the responserequired by the observer to avoid a collision with the threat. While anembodiment of the invention may be designed to operate at radiofrequencies (radar) for use in an aircraft, another embodiment of theinvention may be designed to operate at ultrasonic frequencies for usein a submarine.

Detecting arrangements are known wherein an object can be sensed byreflected radiation for providing intormation as to the location and/ orvelocity of the object relative to the observer. However, in order toprovide the noise immunity required for reliable and accurateinformation the radiation signal-to-noise level heretofore required hasbeen so high as to require relatively complex, bulky, and weighty signaldetection and analyzing equipment. Thus, such equipment has notheretofore been completely successful in applications, such as incommercial aircraft, where bulk, weight, and cost have been primeconsiderations.

Accordingly, an object of this invention is to provide an improvedobject detecting and analyzing arrangement for processing information toprovide the proper maneuver display for collision avoidance.

Another object of the invention is to provide an improved detecting andanalzing method and system for providing a proper maneuver avoidancedisplay and for holding the display for a time period.

A further object is the provision of an improved display arrangementcapable of presenting collision avoidance signals only after thereceived signal has been processed to a level Where unambiguousinformation as to the evasive action required to avoid the mostthreatening of the collision threats is avoided.

Yet another object of the invention is to provide an improved compact,light weight collision threat course prediction arrangement useful inconnection with a radar set operating in a relatively lowsignal-to-noise ratio environment, and wherein the arrangement providesdefinite control information to an annunciator as to the evasive actionrequired on the part of an observer to avoid a collision threat.

The foregoing and related objects are realized in a collision avoidancearrangement embodying a signal analyzing method and system according tothe invention. Firstly, the signal analyzing arrangement is maderesponsive only to signals indicative of certain functions of detectedobject distance and velocity. These functions represent the distance andvelocity value combinations indicative of a prediction that the objectwill reach the general vicinity of the observer at a predetermined timein the immediate future. The distance and velocity value combinationsare chosen such that this itme will be sufiiciently large to allow anescape maneuver should it be determined by subsequent signal processingthat a given object is on a collision course with the observer. To thisend the arrangement of the invention breaks up, into zones, the regionof space to be observed. The velocity of an object in any of the zonesis then analyzed to determine whether it falls within predeterminedbounds, and thus Whether the distance and velocity of the object aresuch that it is a candidate to be further considered as a collisionthreat. The analyzing of the object by distance and velocity zonesallows a relatively small frequency bandwidth to be analyzed at a timeso that the eifect of noise on the signal is minimized. In order to evenfurther minimize the effect of noise, the more distant zones areexamined on a relatively low bandwidth basis as, for example, as todistance and velocity with each of these more distant zones as by beingtracked by a relatively narrow tracking channel, so that noise presentin the zone under consideration, but outside of the particular channelbeing tracked, will not enter the analyzing circuitry.

Secondly, the signals having the aforementioned distance and velocityvalue combinations are then analyzed to determine whether the angulardirection of the object with respect to the observer, relative to itsspeed and distance, is such that it is actually on a collision coursewith the observer; this analysis determines whether there is a need foran evasive maneuver. This directional determination is realized bymeasuring the change in closing velocity between the observer and theobject moving relative thereto, closing velocity being defined as therelative velocity between the observer and an object as measured along astraight line connecting the two. The incoming signals associated witheach object are categorized in accordance with the closing velocity anddistance of the object from the observer. The rate of change of closingvelocity is then determined; if the distance and closing velocityassociated with an object fall within a predetermined range and the rateof change in the value of the closing velocity is below a predeterminedvalue (for a given value of closing velocity)-the range covering thosedistance and associated velocity change which together indicate that theobject will substantially meet the observer-an indication of the needfor an evasive maneuver, and the type of maneuver required, isindicated.

The analyzing arrangement according to the invention also includes aprovision for the indication of the required evasive maneuver forgreatly distant objects, for example distant mountains, and forexceedingly close objects whose very proximity constitutes a collisionthreat.

The invention is described in greater detail in connection with theappended 12 sheets of drawings wherein like reference characters referto like parts or circuit arrangements, and wherein:

FIGURE 1 is a diagram illustrating the diiferent regions of spaceobserved by portions of the system of the invention in detecting andanalyzing the presence and course of objects relative to the observer;

FIGURE 2 is a block diagram of the general arrangement of a systemconstructed in accordance with the invention;

FIGURE 3 is a diagram of the antenna, transmitter, and receiver portionsof the system of FIGURE 2;

FIGURE 4 .is a diagram primarily concerned with the far and near rangeguard channel portions of the system of FIGURE 2;

FIGURES 5a and 5b are, respectively, diagrams of the far and near rangedetection and analyzing portions of the system of FIGURE 2;

FIGURE 6 is a diagram of collision predictor portions of the system ofFIGURE 2;

FIGURE 7 is a pictorial illustration of an aircraft cockpit annunciatorarrangement useful in connection with the system of FIGURE 2;

FIGURE 8 is a schematic illustration of a direction sensor circuituseful in the arrangement illustrated in FIGURES 4 and 6;

FIGURE 9 is a schematic illustration of a direction sorting circuituseful in a portion of the system of FIG- URE 4;

FIGURE 10 is a schematic illustration of a power supply arrangementuseful in the system portion of FIG- URES a and 511;

FIGURE 11 is a graphical illustration of the distance and trackingarrangements provided by the system of FIGURE 2 as applied to theregions of space depicted in the illustration of FIGURE 1;

FIGURE 12 is a chart showing the various distance, velocity,acceleration, and tracking arrangements used in practicing oneembodiment of the invention; and

FIGURES l3 and 14 are graphical representations of field patterns takenin, respectively, azimuthal and elevational planes of the antennas usedto produce the detection regions designated zones in FIGURE 1.

In general In order to more clearly understand the operating principlesof the present invention and the novel features of advantage ofieredthereby in processing, analyzing, and/or transducing intelligencesignals representative of such data, as for example the distance andvelocity of a detected target, description will now be undertaken of acollision warning system suitable for use by aircraft. The system to bedescribed will be based upon the use of Doppler radar techniques,although the present invention is in no way limited in value toassociation with such techniques.

In such a system, in order to develop information as to the evasivemaneuver required to avoid a collision, the volume of space observed bya system in which the invention finds especial utility is divided intothree categories, each category being used to warn of a different typeof collision threat. Specifically, as illustrated in FIGURE 1, thevolume of space observed by the system is divided into: (a) a far-rangeregion, herein referred to as a far range guard ring, for detecting thepresence of large terrain obstacles at an appreciable distance in adirection forwardly of the observer aircraft, say at a distance range offrom 10,000 to 12,500 feet from the aircraft; (b) an intermediate region(with which the invention is especially concerned) herein referred to asbeing made up of a number of collision detection zones (each having adifferent association of distance and velocity ranges) for detecting thepresence of those objects, forward of the observer aircraft, that may beon a collision course with the aircraft and analyzing the return signalsproduced by these objects to predict their course, the collisiondetection zones extending for a distance range of say 1,750 to 10,000feet from the aircraft; and (c) a near range region, hereinafterreferred to as a near range guard ring, for detecting the presence ofobjects at relatively short distances, say at distance of less thanabout 500 feet from the aircraft, regardless of direction.

With respect to the far range guard ring (zone 6 in FIGURE 1), signalsreflected from a terrain obstacle appearing in a prescribed frontalportion of this guard ring are purposely received at two differentforwardly directed antenna systems portions on the protected aircraft.As a result, a phase difference is introduced between the signals atthese two portions, the phase difference being measurably related to thedirection of the obstacle relative to the system. The two out-of-phasesignals are then applied to a far range guard channel that providesdirectional information, based on the phase angle between the signals,for avoiding the obstacle. Range gating is used to define the limits ofthe far range guard ring.

In connection with the near range guard ring, signals returned from anobject in this region, regardless of the direction or velocity of theobject, are received by one of two hemispherically directed antennas,the antennas being designed to receive return or echo signals from,respectively, close objects on opposite sides of the observer aircraft.These opposite sides may be, for example, in directions upwardly anddownwardly of the aircraft. Range gating is used to make this guard ringresponsive only to objects within a prescribed close distance from theaircraft, say within 500 feet of the aircraft. The general direction ofa detected object relative to the aircraft is determined by producing,in response to the detection of an object in the near range guard ring,a biasing signal which is thereafter used to effectively cut out theservice of one of the two antennas. If signals continue to be passed tothe guard channel, the echo signals must therefore be coming from anobject in the direction covered by the antenna whose service was not cutout. Conversely, if no signals are thereafter passed to the guardchannel, the echo signals must necessarily be originating in thedirection guarded by the antenna whose intercepted signals have been cutout.

As to the intermediate or collision detection zones 1 to 5 of theradiated electromagnetic field: An arrangement according to anembodiment of the invention useful in protecting aircraft is based onthe principle that collision warning information is needed only in casesof imminent or near imminent collision, for example 15 seconds before athreatened collision. The provision of a greatly longer warning timeperiod makes for a less accurate and less reliable arrangement since thedirection and velocity of either the observer or object to be detectedare more likely to change from a collision to non-collision course, orfrom a non-collision course to a collision course, within a greaterwarning period. This greater chance for change in course would give riseto a larger number of false collision warning alarms. Likewise, assumingthe maximum effective closing velocity of an object to be fixed, theadoption of a warning time greatly in excess of, 15 seconds for example,greatly increases the signal-to-noise problem. Receiver range must beincreased and/ or transmitter power increased thereby making theequipment more bulky, weighty, and costly.

Signals returned to the observer aircraft from collision threats, suchas other aircraft traveling at relatively high speeds through one ofthese zones, have characteristics that are used to determine Whether ornot the collision threat and the observer aircraft are on a collisioncourse. These characteristics are used to categorize collision threatsas to their closing velocity, distance, rate of change of closingvelocity, and direction relative to the protected aircraft. One suchcharacteristic, namely Doppler frequency shift, is proportional toclosing velocity between the system and the detected aircraft, closingvelocity being defined as the relative velocity between the systemaircraft and the threatening aircraft measured along a straight lineconnecting the two. The system illustrated by way of example includes acollision channel that provides a signal at the Doppler shift frequency,thereby providing information for categorizing collision threats interms of their closing velocity. On the other hand, the distance of thedetected collision threat is determined by range gating. If the closingvelocity and distance of the collision threat in any of the collisiondetection zones 1 to 5 fall within predetermined distance and velocitycombinations which together would cause the object to arrive at thelocation of the observer aircraft within the given time interval, and ifthe rate of change of velocity is zero or close to zero, an indicationof the danger is provided. The direction of approach of the collisionthreat is determined by either of the two direction detection methodsdescribed.

By breaking up the intermediate region distance and velocity parametersto be sensed by the system into relatively narrow divisions, zones 1 to5, the effect of noise on the system is reduced. The effect of noise iseven further minimized by time sampling some of the more distance zones(far range detection zones 4 and 5) in order to restrict the amount ofinformation that must be analyzed by any portion of the system at onetime. In accordance with the present invention, the velocity of theobject under observation is broken down into velocity groups correlatedwith different distance zones so that only critical combinations ofvelocities and distances are noted by the system of the invention withan attendant increase in noise immunity.

Furthermore, in the system the velocities of the object are analyzed todetermine Whether their velocities are such as to have them notconstitute a danger in view of a course which will bring them close to,but still at an appreciable distance from the observer so as not toconstitute an actual collision threat.

By means of the foregoing, it has been found that an aircraft collisionavoidance system may be made, embodying the present invention, that willhave a relatively small Weight and bulk. Since the method and system ofthe invention finds especial utility in connection with a particularvariety of Doppler radar aircraft collision avoidance system (forexample of the general type described in greater detail in copending US.patent application Serial No. 587,768, filed May 28, 1956, entitledCollision Indication System, by Emory Lakatos et -a1., and assigned tothe same assignee as the present invention), the analyzing arrangementof the invention will be described in connection with this general typeof collision avoidance system.

The collision avoidance system Referring now to FIGURE 2, a collisionindication system of the type referred to is here illustrated broadly inblock diagram fashion. A Doppler radar transmitter as a whole isdesignated by numeral 10, the entire system receiver is designated by apair of receiver channels 111: and 11b, and the antenna system, whichperforms the dual function of radiating the signals generated by thetransmitter and relaying return or echo signals to the receiverchannels, is generally designated 12. The antenna network 12 radiatesinto space pulsed energy generated by the transmitter 10. A pair of T-R(transmitreceive) switches 21a and 21b are connected to the transmitter10, and a pair of directional couplers 22a and 22b (in the antennasystem 12) are connected in series between the T-R switches and two setsof radiation pattern producing and receiving means or antennas 23a, 23b,24a, and 24b (FIGURES 2 and 3).

The system shown makes use of two distinct types of sensing regions forproviding information relative to the presence of objects in theregions. The first of the sensing regions extends for an appreciabledistance outwardly from the observation aircraft along lines generallyconcurrent with its path of travel, or generally forwardly for theaircraft, while the second of the regions extends for a short distancein a generally spherical pattern around the aircraft, with the first andsecond sensing regions being established by the separate pairs ofantennas 23a and 23b, and 24a and 24b, respectively. The first set ofantennas is made up of a pair of forwardly directed antennas 23a and23b, and the second set of antennas comprises a set of hemisphericallydirected antennas 24a and 24b with the antennas of the second settogether forming a substantially spherical antenna pattern. Theforwardly directed antennas 23a and 23b comprise the primary radiationpattern producing and receiving means for the aircraft to be protectedand the hemispherically directed antennas 24a and 24b comprise thesecondary radiation pattern producing and receiving means for theaircraft.

Referring to FIGURE 13, there is illustrated therein elliptically shapedcurves 62 and 63 which represent the individual field patterns for theforwardly directed antennas 23a and 23b (FIGURE 3), respectively, takenin an azimuthal plane, the patterns being typical of those obtained froman antenna backed by a reflector such as the antennas 23a and reflector28. The combined field pattern for the forwardly directed antennas 23aand 23b, obtained by adding the individual field patterns 62 and 63, isgenerally designated 64 and, as shown in FIGURE 13, generally resemblesa semicircle. The elevational field pattern contour of the forwardlydirected antennas is shown in FIGURE 14 and is generally designated 65.The elevational field pattern is generally cigarshaped, that is, ofrelatively narrow beam width, and includes a plurality of shorter lobeswhich are useful in providing additional vertical coverage not obtainedfrom the main lobe 65.

Referring back to FIGURE 2, in the time periods during which signals arereceived by the antenna system 12 the T-R switches 21a and 21b areoperative to pass received signals into the receiver channels 11a and11b. The signals processed in the receiver channels 11a and 11b are fedto the far range guard processing channel 31, the near range guardprocessing channel 32, and the collision information analyzing channel33. The receiver channels 11a and 11b feed their outputs directly to thefar range guard processing channel 31 while the near range guardprocessing channel 32 and the collision channel 33 each receive the sumof the signals provided by the receiver channels 11a and 11b, the sum ofthe signals received by the receiver channels 11a and 11b being providedby an adder circuit 42.

The signals fed into the far range guard channel 31 are applied to rangegates 45 for passing only those signals indicative of objects detectedwithin the distance range limits (for example from 10,000 to 12,500feet) to be processed by this channel 31. The signals passed through thefar guard range gates 45 and then compared with each other in a phasedirection sensor 47 for determining whether the detected object lies ina right or left portion of the far range guard ring (FIGURE 1). Thephase direction sensor 47 gives an indication of the presence of anobject and its relative right or left position in the far range guardring by actuating an appropriate indicator in a bank of annunciators 36.

The signal fed into the near range guard processing channel '32 from theadder 42 is first subjected to a near guard range gate 53 fordetermining whether the detected object lies within the near range guardring, for example a distance of 500 feet in all directions about theobserver aircraft. A signal indicative of the presence of such an objectis fed into a direction sensor 54 which determines the direction of thesensed object relative to the observer aircraft by momentarily passing asignal to a receiver channel disabling means 49, a circuit thatmomentarily cuts off one receiver channel 11a. This effectively removesfrom service the antennas associated with the primary and secondaryradiation pattern means 23:: and 24b. If the direction sensor 54continues to receive a signal indicative of the presence of an object,an indication is fed to the annunciators 36 that the detected objectlies in the directions handled by the other receiver channel 11b.Conversely, if the receiver signal is cut off, an indication is giventhat the object lies in the directions handled by the receiver channel11a subjected to the channel disabling means 49.

The signal fed into the collision channel 33 from the adder 42 isapplied to two separate sets of distance and velocity range gates. Theserange gates are included in the circuitry referred to in FIGURE 2 by thelegends Doppler analyzer far range detection zones and Doppler analyzernear range detection zones 82; These range gates (explained in detail inconnection with FIG- URES 5a and 5b) d'etect combinations of distanceand velocity ranges of detected objects which will arrive at thelocation of the observer aircraft within a predetermined time, say 15seconds, after the receipt of the signals. If either of the distance andvelocity analyzing circuitry 80 or 82 detects such a distance andvelocity combination it feeds a signal indicative of this fact to itsassociated collision predictors 84 or 86, respectively. These collisionpredictors 84 and 86 study the return signals to determine whether theobject and observer will miss each other by a safe margin, say 500 feetor more, even though the object and observer are on a course that willbring them in the same general vicinity. This miss determination is madeby analyzing the rate of change of frequency, or Doppler shift, of thereturn signal. A rate of frequency change more than a predeterminedminimum (this predetermined minimum being a function of the distance ofthe object and is thus different for each of the direction zones 1 to 5referred to) is indicative of the fact that, While the object andobserver are moving generally toward each other, they will miss eachother by at least the predetermined safe distance. On the other hand, ifno frequency change is observed, or if the rate of frequency change isless than the predetermined minimum for the particular distance zoneinvolved, the determination is made that the object and observer are ona collision course, or that the object and observer will miss each otherby less than the safe distance referred to.

Finally, the output from the collision predictors 84 and 86 are fed to,respectively, direction sensors 88 and 90 for determining the directionof approach of the object relative to the observer (that is, whether theobject is approaching from the right or from the left) so that thecollision warning information applied to the annunciators 36 may takethe form of an indication of the course of action to be taken by theobserver to avoid the threatened collision. The direction sensors 88 and90 operate in the manner described above with respect to the directionsensor 54 of the near range guard processing channel 32, namely, bycausing the channel disabling means 49 to momentarily disable onereceiver channel 11a and observing whether the signal being analyzedcontinues to be received.

The collision avoidance system will now be discussed in greater detail.Referring to FIGURE 3, the pulsed transmitter includes a crystaloscillator 13 which genates a stable signal at a basic referencefrequency. In the embodiment of the invention shown by way of examplethe frequency of the oscillator signal is 60 megacycles. The oscillator13 has two output terminals 92 and A 94, the first terminal 92 of whichis connected to a frequency multiplier circuit made up of two multiplierstages 14a and 14b which increase the frequency of the oscillator signalto some predetermined higher value. In the present case the oscillatorsignal is multiplied upwardly a number of times from 60 megacycles to afinal 2,940 megacycle signal at the output from the second stage 14b.The second of the two oscillator output terminals 94 is connected to aDoppler reference amplifier 96 that is fed through a line M to a Dopplerdetector or mixer 98 (FIGURE 4) described below.

The output from the second frequency multiplier stage 14b is connectedto a driver 100 which in turn is connected in driving relation to apower amplifier 15 through one terminal '104 of two input terminals 104and 106 to the power amplifier. The output signal from the poweramplifier 15 is applied through a circuit connection 116 to atransmitter power divider which equally divides the power delivered bythe power amplifier 15 to the two T-R switches 21a and 21b. The secondinput terminal 106 to the power amplifier 15 is connected to a pulsemodulator circuit 16 that generates a train of pulses used to modulatethe 2,940 megacycle continuous wave signal applied to the poweramplifier 15 by the second frequency multiplier stage 14b.

The pulse repetition rate of the pulse train applied by the pulsemodulator circuit 16 is determined by a 40 8 kilocycle pulse repetitionrate oscillator 17 connected to the pulse modulator 16. Consequently,the basic transmitter pulse repetition rate is 40 kilocycles. However,the pulse repetition rate is varied in a prescribed manner from 40kilocycles to guard against interference from other signals radiated atsubstantially the same carrier frequency and pulse repetition rate, asfrom a second similarly equipped aircraft in the vicinity of theobserver aircraft. This pulse rate variation is realized by a jitterfrequency modulator circuit 18, actuated by a jitter oscillator 108,connected to the 40 kilocycle oscillator 17. A portion of the jittermodulated 40 kilocycle output from the 40 kilocycle pulse repetitionrate oscillator 17 is fed via a line L to a sawtooth ramp generator 128(FIGURE 4). The portion of the output pulses from the oscillator 17,these spiked pulses being illustrated at numeral 130 in FIGURE 4, areused to develop a sawtooth ramp signal having a periodicity of 40kilocycles per second jitter modulated at the same rate as that of thetransmitted pulse. Since the transmitter and receiver jitter modulationis derived from the same source, the range gating of the receivedsignals, which is determined by the spiked pulses 130 applied to thesawtooth ramp generator 128, is in synchronism with the transmittedpulses in spite of the jitter modulation provided by the jitteroscillator 108.

Considering the component circuits of the transmitter 10 in greaterdetail, the crystal oscillator 13 may be an electronically coupledcrystal oscillator of the type that is well known in the electronicarts, the crystal being preferably contained in a crystal oven tominimize the effects of temperature change, which change may causeundesirable deviations from the frequency at which the oscillator isdesigned to operate, 60 megacycles in the present case.

The first frequency multiplier circuit stage 14a may be made up ofseveral tandemly connected doubler and tripler stages that successivelymultiply the frequency of the oscillator signal until a final frequencyis obtained from the first multiplier stage 14a that is 48 times that ofthe original 60 megacycle signal, namely, 2880 megacycles. The signal atthis 2880 megacycle frequency is fed via a line to the receiver portionsof the system to provide the required intermediate frequency signal, aswill be explained below. The second frequency multiplier circuit stage14b includes a mixer circuit for the purpose of producing a 2940megacycle signal by heterodyning the 2880 megacycle signal received fromthe first multiplier circuit stage 14a against a 60 megacycle signalgenerated by the oscillator 13. It is by this means that the 2940megacycle carrier frequency is produced. Since conventional doubler,tripler, and mixer circuits may be used in the multiplier circuit stages14a and 14b, no further description of them is deemed necessary.

The power amplifier 15 may be a multi-cavity klystron or similarmicrowave frequency tube which, when actuated, supplies the requiredpower to the antenna system 12. Various types of klystrons and othermicrowave tubes that may be utilized herein are shown and describedthroughout volume 7 of the MIT Radiation Laboratory Series, entitledKlystrons and Microwave Tubes, by Donald R. Hamilton, Julian K. Knippand I. B. Horner Kuper, published in 1948 by the McGraw-Hill BookCompany, Inc., New York.

The jitter frequency modulator circuit 18 basically comprises twocircuit, one a sawtooth generator circuit that, as the name implies,produces sawtooth signals, and the other a circuit that produces asignal whose frequency varies with the voltage of the sawtooth signalproduced by the sawtooth generator. A reactance tube modulator iscustomarily used for the latter circuit and is preferred herein, thereactance tube modulator operating to vary the inductance of afrequency-determin ing tank circuit of an oscillator, thereby producingthe desired frequency modulated signal.

The 40 kilocycle or pulse repetition rate oscillator 17 s,oos,194

generally comprises a standard type of audio oscillator adjusted toprovide a 40 kilocycle signal, the signal being produced at 40kilocycles by means of a tank circuit tuned to that frequency. Thereactance tube modulator heretofore mentioned is customarily connectedin parallel with the 40 kilocycle tank circuit and continuously tunesthe tank circuit over a predetermined, usually narrow, range offrequencies by varying the value of inductance in the tank circuit inaccordance with the voltage amplitude of the sawtooth signal generatedin the frequency modulator circuit 18. The pulse repetition rateoscillator 17 may also include a 40 kilocycle magnetic chopper amplifierthat takes the sinusoidal output of the audio oscillator and produces asquare wave signal therefrom.

The pulse modulator :16 preferably includes a lumped constanttransmission line type of pulse-forming network, a discussion of whichis presented on pages 175 through 224 of volume 6 of the aforementionedMIT Radiation Laboratory Series, entitled Pulse Generators, by G. N.Glascoe and J. B. Lebacqs, published in 1948 by the Me- Graw-Hill BookCompany, Inc., New York. The pulse modulator 16 serves the purpose ofconverting the abovementioned square wave signal from the oscillator 17into a train of unidirectional rectangular pulses of proper duration andenergy content, the pulse train being applied to the power amplifier 15to recurrently activate it for controlling the release of the pulses ofenergy from the transmitter 10. I

The antenna network 12 radiates into space the pulsed energy generatedby the transmitter 10. The antenna system 12 receives the transmitterenergy through the transmitter power divider 20 that equally divides thetransmitter output power and passes the power through the T-R switches21a and 21b into the antenna couplers 22a and 22b and to the forwardlydirected and hemisperically directed antennas 23a and 23b, and 24a and2412, respectively. Attenuators 112 and 114 may be inserted,respectively, between the couplers 22a and 22b and their respectivehemispherically directed antennas 24b and 24a to assure that thehemispherically directed antennas operate at an appreciably lower powerthan the forwardly directed antennas in spite of the fact that all ofthe antennas are energized from the same energy source, the singletransmitter 10.

More specifically, the transmitter power divider 20 is a 3-ended devicewhich equally divides the signal power applied to its input end 116 toproduce therefrom two identical signals of equal power at its outputends 118 and 120. In the present instance the input end 116 of thedivider 20 is connected to the power amplifier 15 of the transmitter andthe two output ends "118 and 120 are connected to the T-R switches 21aand 21b, respectively. Several power dividers that may be used in thepresent embodiment are shown and described on pages 516 through 528 ofvolume 9 of the aforementioned MIT Radiation Laboratory Series, entitledMicrowave Transmission Circuits, by George L. Ragan, published in 1948by the McGraw-Hill Book Company, Inc., New York.

The T-R switches 21a and 21b are alsoconnected to, respectively, crystalmixer circuits 37 and 38 of the receiver channels 11a and 11b (FIGURE 2)and to the direction couplers 22a and 22b, the T-R switches serving toalternately connect the transmitter 10 and the receiver channels to thedirection couplers and, therefore, ultimately to the various antennas23a, 23b, 24a and 24b. A number of TR switches that may be employed inthe embodiment illustrated are described in pages 226 through 375 ofvolume 14 of the MIT Radiation Laboratory Series referred to, entitledMicrowave Duplexers, Louis N. Ridenour, published in 1948 by theMcGraw-Hill Book Company, Inc., New York.

The directional couplers 22a and 22b are devices that are generallysimilar to the transmitter power divider 20 in that each coupler is alsoa 3-ended device, but with the difference that the directional couplersdivide the output power therefrom in some unequal prescribed ratio withrespect to the power applied to its input end. As illustrated in FIGURE3, the input ends 122a and 12% of, respectively, the directionalcouplers 22a and 22b are connected to the T-R switches 21a and 21b,respectively, and the two output ends 124a and 126a of one directionalcoupler 22a are respectively coupled to the right and upwardly directedantennas 23a and 24b, and the two output ends 124b and 12Gb of the otherdirectional coupler 22b are respectively coupled to the left anddownwardly directed antennas 23b and 24a. As indicated above, theconnection between each of the couplers and its respective hemisphericalantenna may be made through an attenuator for reducing the signalstrength of the signal received from these antennas. While the right andupwardly directed antennas are here illustrated as being coupledtogether, it will be understood that, instead, the right and downwardlydirected antennas may instead be coupled together (with the left andupwardly directed antennas coupled together). Since the forwardlydirected antennas 23a and 23b are required to receive intelligiblesignals over a distance appreciably greater than that of thehemispherically directed antennas 24a and 24b, the directional couplers22a and 22b are constructed to couple appreciably more energy betweenthe T-R switches 21a and 21b and the forwardly directed antennas 23a and23b than between the T-R switches and the hemispherically directedantennas 24a and 24b. If the directional couplers are such that therequired ratio of energy is distributed between the forwardly andhemispherically directed antennas without the use of attenuators, theattenuators 112 and 114 (FIGURE 2) may be dispensed with, the couplersthen being directly coupled to the hemispherical antennas. Directionalcouplers that may be adapted for use in the embodiment illustrated areshown and described in detail on pages 854 through 987 of volume 11 ofthe MIT Radiation Laboratory Series, entitled Technique of MicrowaveMeasurements, by Carol G. Montgomery, published in 1947 by the McGraw-Hill Book Company, Inc., New York.

Considering the antennas in further detail, the forwardly directedantennas 23a and 23b are preferably broadside arrays of dipoles and arealso preferably oriented with respect to each other so as to form adegree angle therebetween, as indicated by dashed lines 25 and 26.Dashed line 27 is a center line and divides the angle formed by theantennas 23a and 23b into two smaller equal angles, each smaller angletherefore being substantially 45 degrees. Immediately behind theantennas 23a and 23b, and insulated therefrom, are a pair of reflectors28 and 30, respectively, the reflectors being oriented in the samemanner as the antennas, as shown in FIGURE 3, and being spacedapproximately onequarter wave length therefrom to produce unidirectionalfield patterns.

Due to the presence of the reflecting shields 28 and 30, for allpractical purposes only signals reflected from a target positionedwithin the 90 degree frontal angle defined by dashed lines 25 and '26will be received by both forwardly directed antennas 23a and 23b. Inother words, signals reflected from a target lying in the angle definedby the right antenna 23a and dashed line 25 will be reflected by theshield 28 and, therefore, for all practical purposes the reflectedsignal wave front will never reach the left antenna 23b. Likewise, fortargets in the angular region defined by the left antenna 23b and dashedline 26, wave fronts of signals reflected from these targets will neverreach the right antenna 23a.

With respect to the hemispherical antennas 24a and 24b, these antennasare used to obtain hemispherical coverage above and below the aircraftand, although truly hemispherical radiation patterns are not obtainablein practice, they can be approximated by making the antennas 24a and 24bflush-mounted and cavity-backed circumferential slots locatedapproximately in the fuselage of the aircraft bearing them. While thehemispherical antennas have been described as being oriented upwardlyand downwardly of the protected aircraft it will be ap preciated thatthese antennas may instead be oriented to cover hemispherical regions tothe right and left of the observer aircraft. In such a case the righthemispherical antenna is preferably connected to the right forwardlydirected antenna and the left hemispherical antenna to the leftforwardly directed antenna.

The receiver channels 11a and 11b (FIGURE 2) comprise the mixer circuits37 and 38 (FIGURE 3), each mixer circuit having first and second inputterminals 130a and 132a, and 13% and 132b, respectively. The first inputterminals 130a and 13Gb of each of the mixer circuits 37 and 38 areconnected to the T-R switches 21a and 21b, respectively, and the secondinput terminals 132a and 13% of these mixers are connected,respectively, to the two output ends 134a and 134b, respectively, of areceiver power divider 39 whose input end 136 is connected to receivethe 2880 megacycle signal produced by the first frequency multiplierstage 14a of the transmitter 10. The receiver power divider 39 may besubstantially the same as the transmitter power divider 20 in that itapplies two 2880 megacycle signals of equal power to the mixer circuits37 and 38. The mixer circuits 37 and 38 are connected, respectively, attheir output ends 138a and 138b to a pair of intermediate frequencypreamplifiers 40 and 41, respectively, which in turn are connectedthrough lines J and K, respectively, to a conventional adder circuit 42(FIGURE 4), and to, respectively, right and left far range gates 45a and45b of the far range guard channel 31 (FIGURE 4). One of the outputlines I from the intermediate frequency amplifier 40 includes a seriallyconnected normally closed direction sensing switch 140 (FIGURE 4), thefunction of which 'will be discussed below.

Referring now to FIGURE 4, the far range guard channel 31 comprises thepair of far range gate circuits 45a and 45b connected, respectively, toreceive signals through lines I and K from, respectively, thepreamplifiers 40 and 41 (FIGURE 3). The output ends 142a and 142b of thetwo range gate circuits 45a and 45b, respectively, are connected to thetwo input terminals of a phase direction sensor 50. The phase directionsensor 50 is connected to actuate a right or left far range detector 144or 146, respectively, which determines the directional applied to thedesired indicator 179 in a bank of annunciators 36 (FIGURE 7) throughterminals A1 and A2. Considering the elements of the far range guardchannel 31 in greater detail, the range gate circuits 45a and 45b areeach of the kind that will only pass signals returned from a targetlocated between predetermined upper and lower range or distance limitsfrom the observer aircraft, for example between distance limits of12,500 feet and 10,000 feet. Each of the range gate circuits 45a and 45bcomprises a usual gating circuit, many of which are well known in theart. The gating circuits are normally in an inoperative condition, as bybeing biased beyond cut off, and hence will not normally pass signalsapplied to them. However, in response to a signal 148, applied to thegate circuits 45a and 45b through a line 150, both gate circuits arerendered operable to pass signals which may be applied to them for theduration of the signal 148. As indicated in FIGURE 4, the intervals oftime between the leading edges of successive unblocking signals 148,corresponding to a lower target range limit of 10,000 feet, and thelagging edges of the pulses, corresponding to an upper target rangelimit of 12,500 feet, correspond to the intervals of time during whichthe far range gates 45a and 45b are unblocked. As illustrated in thechart of FIGURE 11, with respect to zone 6 thereof, these 10,000 footand 12,500 foot distances correspond to, respectively,

12 time periods of a little more than 20 and 25 microseconds after thetransmission of a pulse by the antenna system 12.

The unblocking signals 148 applied to the far range gates 45a and 45bare produced through the agency of a conventional blocking oscillator152 triggered by pulses 154 received through a line 156 connected to aconventional pickoif circuit 158. The pickoff circuit 158 re ceives asawtooth signal 160 from the sawtooth ramp generator 128 and generates avoltage spike, signal 154, at a time interval established by themagnitude of the direct current threshold potential applied to thepickoif circuit 158. This direct current threshold potential is appliedto the pickotf circuit 158 through a terminal 162.

With respect to the phase direction sensor 50, this is a type of circuitthat produces an output voltage whose polarity is indicative of thephase angle between the two signals applied thereto, a number of suchcircuits being well known in the art. A number of phase detectorcircuits that may be adapted for providing the phase direction sensingrequired of the far range guard channel 31 are shown and described onpages 511 to 524 of volume 9 of the MIT Radiation Laboratory Series,entitled Waveforms, published in 1949 by the McGraw- Hill Book Company,Inc., New York. As indicated be fore, the phase direction sensor 50operates on the principle that signals reflected from a terrain obstacleappearing in'a prescribed frontal portion of the far range guard ringare purposely received at the two separate forwardly directed antennas23a and 23b. As a result, a phase difference is introduced between thesignals at these two antennas, the phase difference being measurablyrelated to the direction of the detected obstacle relative to theseantennas. The polarity is a measure of the direction of the obstaclerelative to the observer aircraft.

The near range guard channel 32 receives a signal from the antennasystem 12 (FIGURE 2) through an intermediate frequency amplifier 43,connected to the added circuit 42, and through a video detector circuit52. The output end 164 of the video detector circuit 52 is connected tothe first terminal 166 of two input terminals of a near range gatecircuit 53, which circuit is basically the same as either of the farrange gate circuits 45a and 45b of the far range guard channel 31. Thesecond input terminal 168 to the near range gate circuit 53 is connectedto a blocking oscillator 170, controlled by a pickotf circuit 158. Theblocking oscillator 170 and pickolf circuit 158 are basically the sameas the blocking oscillator and pickolf circuit 152 and 158,respectively, of the far range guard channel 31 but with the diflerencethat the near range guard channel circuits 158 and 170 have voltage andtiming relationships to open the near range gate 53 substantiallysimultaneously with the transmission of a signal by the transmitter andto close the near range gate at a time interval corresponding to thereception of signals reflected from a target at a distance of about 500feet from the antenna system. As illustrated in the portion of the chartof FIG- URE 11 referred to in connection with zone 0, the near rangegate remains open for a time duration of about 1 microsecond.

The direction sensor circuit 54 (FIGURE 2) of the near range guardchannel 32, described in detail below in connection with FIGURE 8,operates by momentarily cutting out one of the near range antennas fromthe receiving channel. If signals continue to be passed to the receivingchannel, the signals must be coming from an object in a directioncovered by the antenna whose signals were not cut out. Conversely, if nosignals are received after the cut out of one of the antennas, thesignals must necessarily be originating in the direction guarded by theantenna whose intercepted signals have been cut out. To this end thenear range gate output terminal 172 is applied to a detector circuit 174and to a one-shot, 5 second multivibrator 176. The output of thedetector circuit 174 momentarily sends an actuating pulse through line178 to the normally closed direction sensing switch 140 for opening thecircuit to signals received from the upwardly directed hemisphericalantenna 24b. (The signals received by the near range guard channel 32from the right forwardly directed antenna 23a are also cut ofi but, aswill be explained below, direction sensing information is not requiredwith respect to near range information received by the forwardlydirected antennas 23a and 23b.) If an output signal from the detectorcircuit 174 persists, the direction sensor 54 produces an indicatingpotential on line A4 producing a signal indicative of the directionalevasive action required to avoid a collision with the detected object.Thus, an appropriate signal is indicated on the near range panel portion180 (FIGURE 7) of the annunciators 36. The one-shot, 5 secondmultivibrator 176 actuates the panel portion 180 and maintains anindication on the panel for the 5 second duration of the multivibratoroutput.

Referring to FIGURE 8, the direction sensor 54 will now be discussed ingreater detail. As indicated above in connection with FIGURE 4, theoutput signal from the near range gate 53 is applied simultaneously,through line 172, to the 5 second multivibrator '176 and the thresholddetector 174, the latter comprised essentially of a threshold circuitfor providing a signal output only in the absence of a signal having anamplitude greater than that of a predetermined minimum value.

The 5 second multivibrator output is fed through a double pole, normallyopen relay 175, closing both sets of contacts 177 and 179 thereof. Oneset of contacts 177 feeds current from a direct current source throughone set of contacts 187 of a double pole relay 191 momentarily actuatinga right annunciator 180 of the annunciator bank 3 6 (FIGURE 7), and theother set of normally open contacts 179 feeds a signal into an and gate189. The multivibrator output is also fed through a .1 second, normallyopen, one-shot multivibrator circuit 173 for momentarily producing asignal through a line 178 to the antenna disabling direction sensingswitch 140 (FIGURE 4). The signal to the direction sensing switch 140cuts out one of the antenna circuits from the direction sensor 54. Ifthe return signal was being received through an antenna not cut off (saythe down antenna) by the direction sensing switch 140, the normallyclosed contacts 187 continue to pass current and the initially indicatedsignal continues to pass through line A3. Since the normally openmultivibrator 173 operates to cut ofi one of the antenna circuits foronly .1 second, the antenna circuit is quickly restored to operation forthe receipt of signals indicative of collision threats contemporaneouswith the first detected collision threat. However, if the antennacircuit controlled by the direction sensing switch 173 is the circuitthrough which the return signal was received, a threshold informationsignal from the detector 174 (FIGURE 4), is fed into the and gate 189together with the signal being received from the 5 second multivibrator176. The coincidence of these last named signals in the and gate 189causes a current to be passed through the relay 191 closing a normallyopen set of contacts 193 and opening the normally closed set of contacts187. This action effects the actuation of an opposite annunciatorindicator through line A4. The momentary presentation of signal on lineA3, when an ultimate signal is to be presented on line A4, does notprovide directional indication ambiguity in view of the exceedinglyshort period of time that the opposite indication is presented. If thedirectional information is presented by a signal light, the normalthermal lag inherent in light bulbs will prevent the wrong bulb fromeven momentarily lighting.

Collision detecting zones FIGURES 5a and 5b are concerned with an aspectof the over-all system with which the present invention has especialconcern, namely, the collision detection channel 33. In the collisiondetecting channel 33 only those return signals indicative of a distanceand velocity combination that would cause a collision threat to arriveat the location of the observer within a predetermined time, say the 15seconds referred to, are candidates for generating collision warninginformation.

Referring to FIGURES 5a and 5b, it is observed that the output of theintermediate frequency amplifier 43 (FIGURE 4) is conveyed by means of acircuit path 182 to the Doppler detector or mixer 98 previously referredto. The Doppler detector 98 also receives a Doppler reference signalfrom terminal M derived from the output of the Doppler reference orbuffer amplifier 96 (FIGURE 3). The relative velocity of a detectedobject with respect to the observer aircraft can be determined byexamining the frequency of the signal delivered by the Doppler detector98 to its output terminal 190. The frequency of the signal will varyabout a mean of zero frequency since the Doppler detector 98 isessentially a heterodyned mixing device, mixing the output of theintermediate frequency amplifier 43 with the output of the Dopplerreference amplifier as received through line M. If the frequency of thesignal delivered by the Doppler detector 98 is in one direction withrespect to zero frequency, this may arbitrarily indicate a velocityrepresenting a closing of the detected object with respect to theobserver aircraft. If, on the other hand, the output of the Dopplerdetector '98 comprises a frequency on the other side of zero, this willrepresent a velocity in an opposite direction, namely, a departingvelocity.

The apparatus of the invention shown generally in FIGURES 5a, 5b, and 6is employed in order to determine and extract the necessary velocityinformation from the signal delivered by the Doppler detector 98,through a Doppler amplifier 192, and relay this information to the zonalranges 1 through 5 (FIGURE 1) in which a given object is maintaining thevelocities thus detected.

The distance and velocity ranges are determined by the followingconsiderations: As to distance, each distance gate must have a distancerange wide enough to track the detected object at least as long as the 3second observation time referred to, this being the time required toobserve and process information received by the system in order todetermine whether the received signal represents information or noise,and if the signal represents information, to analyze the information.Each of the range gates associated with one of the zones 1 to 5 covers aregion between specified maximum and minimum distance values. Startingwith a maximum range of 10,000 feet for the range gate associated withzone 5, the maximum range of each of the other range gates of zone 4through 1 is smaller than that of the previous higher numbered zonalrange gate by a factor of about .7, as indicated in the table of FIGURE12. The successive minimum distance ranges are also in the same ratio.The distance range dilference between the maximum and minimum distancesettings is the product of the observation time (3 seconds) and thehighest closing velocity associated with the given distance range gate.For example, the highest closing velocity in zone 5 is 1,000 feet persecond. Consequently, the distance range gate associated with this zone5 is constructed to be capable of tracking over a distance range ofthree times 1,000 or 3,000 feet.

As to velocity, the velocity range associated with each distance rangeis determined in the following manner. At the maximum setting of eachdistance range gate a maximum closing velocity is chosen such that thegross time element involved will be 10 seconds, this gross time elementbeing taken as the minimum time that can be allowed for the handling ofinformation and the initiation of the desired warning signal. The ratiobetween the minimum and maximum velocity ranges to be handled within anydistance range gate or zone is then taken as approximately .7 to 1, itbeing observed that in any 15 velocity interval the lower closing ratecorresponds to the greater gross time element.

As indicated in FIGURES a and 5b, the present invention contemplates theuse of two radar Doppler analyzing circuits, one circuit 80 (FIGURE 5a)responsive only to far range information, that is, information receivedfrom objects in zones 4 and 5, and the other circuit 82 (FIGURE 51;)responsive only to near range information, that is, information withrespect to objects in zones 1, 2, and 3. In each of the Doppleranalyzing arrangements of FIGURES 5a and 5b, range gates 194 and 196,and range gates 198, 200, and 202, respectively, are followed byfrequency selective Doppler signal processing circuitry. Inasmuch aseach of the zones 1 through 5 (FIGURE 1) is associated with criticalvelocity and distance combinations which together indicate a potentialcollision, in accordance with the present invention, each of the rangegates 194, 196, 198, 200, and 202, is associated with a particular setof Doppler filters. For example, in FIGURE 5a, the range gate 196 thatpasses signals indicative of signals within zonal range 5 (distanceslying between the range D and D11 which, by way of example have beenindicated in FIGURE 12, correspond to distances of from 10,000 to 7,000feet from the observer aircraft) is controlled by a blocking oscillator240 which in turn is timed by a pickofi circuit 242. In the distancerange D10 to D11 there is established by the above collision threatconsiderations a range of velocities V9 to V10. This range of velocitiesare covered by four velocity gating circuits, often referred to asDoppler or band pass filters 212, 214, 216, and 218, respectively.Likewise, the critical velocities falling in zone 4, distance ranges D8to D9, may be regarded as covering a velocity range of from V7 to V8.This latter velocity range may be covered by fewer Doppler filters, forexample two in number such as 208 and 210, respectively. In the nearrange collision detection zones 1, 2, and 3, processed in circuitryshown in FIGURE Sb, range gates 198, 200, and 202 may be employed forestablishing the zones 1, 2, and 3, respectively. By way of example, twoDoppler or band pass filters have been associated with the output ofeach of these near range gates 198, 200, and 202. Thus, the range gate202 of zone 1 is associated with two Doppler filters 220 and 222,respectively, which taken together cover the critical velocity range ofV1 to V2. With the output of the zone 2 range gate 200, two Dopplerfilters 224 and 226 are employed for giving coverage to the criticalvelocity range V3 to V4. Similarly, Doppler filters 228 and 230 areconnected with the output of the zone 3 range gate 198 to cover thecritical velocity range V5 to V6. Examples of actual values to whichthese velocity and distance designations may relate, are shown in thechart of FIGURE 12.

From the foregoing it is seen that in accordance with the invention eachof the far range and the near range Doppler analyzer circuitry 80 and 82assigns a plurality of Doppler filters to the output of any given rangegate. The present invention takes advantage of this plurality of filtersto reduce the effect of noise, especially thermal noise, upon the outputsignal of the Doppler filters so that the presence and the nature of theinformation signal can be more easily analyzed. Furthermore, inaccordance with the present invention, the far range Doppler analyzingarrangement shown in FIGURE 5a, the analyzer arrangement provided forobjects received in zones 4 and 5, is provided with a novel type ofrange gate which may be descriptively termed an open loop tracking gate.Each of these open loop tracking range gates, which track at a ratecorresponding to the expected rate of distance change of the objectwhose presence is to be detected, are to be distinguished from closedloop tracking arrangements, the latter (exemplified in theaforementioned copending patent application) being effective to track ata rate directly determined by the measured rate of changecharacteristics of the detected return signal. Since the open looptracking arrangement of the invention does not depend on measuredcharacteristics of a detected return signal to directly control the rateof tracking, this tracking arrangement is free of complex servo circuitsand enjoys a relatively greater noise immunity (the presence of noise inthe return signal has substantially no effect on the tracking rate). Onthe other hand, the near range gates associated with the Doppleranalyzer 82, shown in FIGURE 5b, are provided with non-tracking rangegates. The reason for the foregoing is to take advantage of the greatersignal intensity of return radar pulses associated with objects closerto the observer aircraft; thus, each of the nearer range Doppler filtersmay embrace a larger frequency range without greatly reducing thesignal-to-noise ratio for near range signals. On the other hand, for farrange signals, which may be greatly attenuated due to the greaterdistance of the detected object from the observer aircraft, the amountof information passed by each range gate is reduced at any particulartime to that falling within, for example, a 1 microsecond period as willbe made clear in connection with the showing of FIGURE 11 (periods 232of FIGURE 11). If the distance range to which each far range gate 194and 196 is made responsive is changed in timed relation with respect tothe mean velocity of the Doppler signal being received, a given returnsignal may be effectively tracked by the changing distance range. Thus,with respect to the over-all distance range designated zone 5, andillustrated in FIGURE 11, the effective response of gate 232 is movedbetween distance 234 and distance 236, in a direction going from theupper portion of the range of zone 5 to the lower portion of this range.The movement or tracking of the zone 5 range gate 232 through zone 5 isat a rate corresponding to the mean velocity expected in the range gatebeing tracked. For example, as illustrated in FIGURE 11, for a meanvelocity m, at time interval A, the signal will fall in the center ofthe range gate and remain at the center of the range gate through thetracking as illustrated by successive time intervals B and C Wheresignal m is shown to remain in the center of the range being tracked.For objects having velocities slightly greater or slightly less than themean velocity, the range gate 232 will still track these objects sincein accordance with the present invention the l microsecond range gate iswide enough to accommodate these slightly different velocities. Thus,for example, if a faster than mean velocity object, indicated at points1, is passed through the range gate 232, the return signal 1 will appearat one edge of the range gate during one time period during the trackingand at the opposite edge of the range gate during the last time periodduring the tracking. In actuality, since the open loop tracking referredto is initiated by the detection, in the tracking zone 5, of a signalhaving the distance and velocity parameters to which the zone 5circuitry is responsive, the tracking is preferably initiated by thedetection of the presence of a target at a distance (for example 250feet) greater than the greatest distance (10,000 feet) to which thecircuitry is responsive in order that the detected target fall in thecenter of the 1 microsecond gate at the start of the tracking cycle. Thereason for this is that the substantially different from mean velocitytarget may not be tracked by the tracking circuitry for thesubstantially entire duration of the tracking cycle if the target signalfell at the starting edge of the 1 microsecond gate at the start of thetracking cycle.

Since four Doppler filters 212, 214, 216, and 218, respectively, areprovided for zone 5 (these filters having mean velocities referred to bynumerals 1 through 4 in FIGURE 11) the system of the invention isadapted to simultaneously track at each of the four different meanvelocities so as to accommodate any object velocity within the range ofobject velocities to be handled by zone 5. Typical values for these meanvelocities 1 through 4

